18” Ultra Compact



  18” f4.5 (mirror set shared with my observatory scope)
  Feathertouch Focuser with integrated Paracor, (SIPS)


  Total Weight; about 30kg
  Size; will fit in the boot of my car, (except truss poles!)
  Max eyepiece height; 2.1 metres
  Truss poles; 20mm Al tubes covered in black heat shrink.


  Geared stepper motors driving through toothed belts
  40000 tick encoders 
  Nexus DSC for  scope position readout
  SkySafari for telescope control
  Raspberry PI driving stepper motors via Phidget modules 
  Plate-solving digital finder finishes goto’s to 1 arc min.

Phase 1: I started calling this an Ultralight 'Dobsonian', but during the build realised that it wouldnt be stiff enough unless I significantly increased the weight, especially of the top cage and truss poles. This is mostly due to the weight of the focuser, intergrated Paracor, and 21mm Ethos eyepiece! Weight isnt so much an issue anyway - what is important is that I can get it to UK Star Parties in my car, along with all my camping stuff and sometimes, my wife!  Photos of Phase 1 here.

Phase 2: testing with the new servos showed that the PTFE bearings were giving too much friction. I have now replaced them with ball bearings and now it runs very smoothly and easily. Added the encoders, wifi link and starting to finish off the wiring and installation of ancillaries. Photos of Phase 2 here.

Phase 3: Having proved the drive system to work very well, the decision was made to go into small scale series production! Working with af riend we designed a compact unit built into a high quality aluminium case, laser etched panels and improved user setup and interface. Photos of the production standard are here.

Phase 4: Modifications were undertaken between 2017 and 2019. Changes:

Changed from 'toothed wheel drive’ to ‘toothed belt drive’. Although not as elegant the performance is so much better. At the same time I introduced clutches on both axes. the scope can now be simply pushed to a new location in the sky, and the tracking drive automatically recalculates and takes over at the new position.

Changed the Nexus for a Nexus DSC. This has a built battery, display and is much easier to use.

Added a stalk to mount the Nexus DSC at a convenient height.

Switched to a Samsung OLED tablet (from an iPad) for display & control of the scope drives. Still using SkySafari but the OLED display is truely black and pure red. Maintains dark adaptation even for the most discerning!

Photos are here.

Phase 5: Squeezing everything into my new Mustang was getting tedious, so a rebuild was undertaken to make the scope more compact when being transported, and a bit stiffer in winds. 

I had found a very good and reasonably priced 2” filter wheel on eBay and managed to fit this neatly direclty on to the back of the SIPS Feathertouch focusser. 

The latest version of the 18” UC is shown here.

Recently I’ve added a digital finder scope, and ‘eFinder’. This is described here. It automatically improves GoTo accuracy to about 1 arc minute with respect to real sky RA & Dec.

I have decided that the current design no longer should be called a Dobsonian. It is so far from John Dobson’s original concept both mechanically and with the addition of tracking, GoTo and eFinder. Its an AltAz Scope!

© AstroKeith 2022